Reference U.S. Pat. No. 5,546,441 relates to an X-ray system including an X-ray generator for operating an X-ray tube having a cathode which can be heated by the filament current, comprising means which are operated in an exposure mode for boosting the filament current to a boost value for the duration of a boost time, and means operated in the exposure mode to decrease the filament current and to switch on the tube voltage. The X-ray generator has a special mode in which the filament current is boosted to the boost value while the tube voltage is switched on, means are provided for measuring the tube current flowing in the special mode, a first memory is provided for storing the temporal variation of the measured tube current, and means are provided for deriving a boost time from the temporal variation stored in the first memory. A second memory may be provided in which stationary values of the filament current are stored for various tube voltages and tube currents, and the means for deriving the boost time performs an access to the first memory and the second memory.